FM Motion Sensor




Introduction: FM Motion Sensor

About: Hi! I'm 14 years old and I like to build electronics, robots, pyrotechnical devices and program all kinds of languages.

This project will show you how to make a wireless motion sensor using an FM transmitter that will send any sound up to a 100meters away when it detects motion and all you need is a FM radio to receive that signal.

This is my first instructable! I hope you like it. enjoy!

Step 1: Parts

list of components...

x1- Motion sensor. "Any motion sensor should work"

x1- 0.1 uF ceramic disc capacitor.

x1- 0.001 uF ceramic disk capacitor.

x1- 1 uF electrolytic capacitor.

x2- 10 pF ceramic disc capacitors.

x1- 22 pF ceramic disc capacitor.

x1- 10 K resistor.

x2- 100 OHM resistors.

x1- 39 K resistor.

x1- NPN transistor / 2N3563 or 2SC945.

A bit of wire.

An 1/8th inch bolt or ferrite screw.


pF=pico farad uF=micro farad.

Electrolytic = polarized or it has a negative and positive terminal unlike ceramic capacitors.

K = 1000- so a 10 K resistor would = 10000 OHMs.

The FM transmitter AKA (bug) is one the easiest / smallest transmitters to build. A few components and some time you can easily build this wireless microphone.

When you build this some instructions will call for "exactly 7.5 turns on a 3/16th inch core using AWG #20 wire" Then what this really means is find an old 1/8th inch bolt, wrap anywhere from 7 to 10 turns around it using whatever size copper wire you have in you junk box. Its that easy! I used a random close to 1/8 inch bolt and just cut the end off as the tuning core for the coil.

The transistor is really not all that critical, and any NPN transistor with a frequency as high as 100 MHz will probably work. Almost all NPN transistors give at least some RF output.

This bug can run on 1.5 to 9 volts max. FYI I haven't tried more the 9 volts. The longer the antenna the further the transmission of the audio signal.

Step 2: The Transmitter

The schematic shown above shows where to solder the components. I built the whole thing on some pref board. Usually building a transmitter with wires or jumper cables flying around the board would be a big "no no" for RF circuits but thankfully with most FM bugs it doesn't really matter. If you'r a top-knotch solderer you could probably bring this down to the size of a thumb nail by using super small surface mount component. I will show how to do this in another instructable. Also make sure you solder the transistor in right because I soldered it backwards and it just won't work that way.

Step 3: Put It All Together

Once you have built the transmitter we can get on to putting it all together. I hot glued it right on to the side of the motion sensor. FYI the motion sensor only draws 20 miliamps which means low power consumption!

First take the power wires for the transmitter and put the positive in NC and the negative in C ports. They are shown in the pics above. If you're motion sensor is not labeled or is different it is very easy to find out which ports are which.

How I figured out which was power is very easy. It should be labeled + or - and to find out which is output "for the transmitter in this case" was also easy. I put my volt-meter into to ports that were right next to each other and turned the power on to the motion sensor. If you wave you're hand or move it around and you get output on the ports that means you will use these ports for power on the transmitter. If not then keep trying other ports.

Step 4: Find the Frequency

Now that you have put it all together we need to find the frequency. This was one of the toughest parts for me.

I would either get a digital frequency counter or try my method. Turn on the transmitter and attach the mic "if its not already soldered on." Turn on a radio to around 100MHz. Get a plastic screw driver anything thats not metal to turn the metal core in the coil. Talk in to the mic. When you here you're voice on the radio you know it's tuned to the right frequency.

If you still can't find the frequency then try changing the frequency of the radio. If you get a frequency counter that will make every thing easier. You can also use it to see if the transmitter is even working or not.

When it senses motion It should transmit what ever audio signal you want to were ever you have a radio.

Step 5: You're Done!

If you have any questions then please leave a comment below. Thanks for viewing my project and have fun!

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    Pi Thief 00
    Pi Thief 00

    6 years ago on Introduction

    AWESOME PROJECT JOEY-CO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Great first project!!!

    What an awesome gadget, and you did a great job showing the intricate bits of the build. Welcome to instructables Joey-co, I hope we see more from you in the future!